HARP is a NASA/ESTO (Earth Science
Technology Office) funded CubeSat mission under the InVEST (In-Space
Validation of Earth Science Technologies) Program. The HARP CubeSat
mission is a joint effort between the UMBC (University of Maryland
Baltimore County), Catonsville, MD, USU/SDL (Utah State
University/Space Dynamics Laboratory ), North Logan, UT, STC (Science
and Technology Corporation) with HQ in Hampton, VA, and NASA/GSFC
(Goddard Space Flight Center) in Greenbelt, MD. The goal is to deploy
the HARP CubeSat from the ISS. The desired mission life consists of
three months for technology demonstration and an extended science data
period of another seven months, which will total almost a year on
orbit.

The HARP mission is designed to
measure the microphysical properties of cloud water and ice particles
in the atmosphere. HARP is a precursor for the new generation of
imaging polarimeters to be used for the detailed measurements of
aerosol and cloud properties. The HARP payload is a wide FOV (Field of
View) imager that splits three spatially identical images into three
independent polarizers and detector arrays. This technique achieves
simultaneous imagery of three polarization states and is the key
innovation to achieve high polarimetric accuracy with no moving parts.

• Provide opportunities for student research and engineering training in implementing a space mission.

The HARP science goal is to
demonstrate the ability to characterize the micro physical properties
of aerosols and clouds at the scale of individual moderate-sized clouds
for the ultimate purpose of narrowing uncertainties in climate change.

HARP is a potential precursor for the polarimeter in ACE (Aerosol-Clouds and Echosystems) and other future NASA missions.

Spacecraft:

The HARP CubeSat mission will be a
joint effort between UMBC, the PI institution, who will provide the
instrument and characterization and scientific analysis; the Space
Dynamics Laboratory – Utah State University, who will provide the
3U CubeSat spacecraft and mission operations; and the Science and
Technology Corporation, who will lead the science algorithm development
and science application funded by NOAA. NASA Wallops will support
instrument environmental testing, mission operations, and
communications.

The 3U CubeSat is 3-axis stabilized
designed to keep the imager pointing nadir during the data acquisition
period. The hyper-angular capability is achieved by acquiring
overlapping images at very fast speeds.

Launch: The HARP
nanosatellite was launched as a secondary payload on an ISS logistics
mission of Orbital ATK (Cygnus OA-7 , also known as CRS-7)on April 18.
The launch vehicle was Atlas-5 401 of ULA and the launch site was Cape
Canaveral (SLC-41), FL. 5)6)

The ISS orbit has the advantage of
allowing HARP to cross many other Earth Science Satellites (including
Terra, Aqua, Aura, VIIRS on Suomi NPP, CALIPSO, etc.) and produce
intercomparisons and a synergistic use of the HARP data together with
data from these other platforms.

• OPEN (Open Prototype for Educational NanoSats), a 1U CubeSat of UND (University of North Dakota).

• Violet, a 1U CubeSat of Cornell University, Ithaca, N.Y.

• Biarri-Point, a 3U CubeSat
technology mission, a four nation defence related project involving
Australia, the US, the UK and Canada. Biaari is an RF signal collection
mission that can be related to the spot beam mapping mission through
mutual use of GPS signals. 8)9)

• QB50 x 28. Twentyeight
CubeSats of the international QB50 constellation, a European FP7
Project for Facilitating Access to Space and managed by the Von Karman
Institute for Fluid Dynamics in Brussels, were flown to the ISS for
subsequent deployment. The 28 CubeSats of the QB50 constellation were
integrated into 11 NanoRacks 6U deployers. 10)

In addition, four Lemur-2
satellites, operated by Spire Global Inc. of San Francisco , were
launched aboard the Cygnus OA-7 cargo craft to replenish and expand the
company’s constellation dedicated to obtaining global atmospheric
measurements for operational meteorology and tracking ship traffic
across the planet for various commercial applications. The four Lemur-2
CubeSats are mounted externally to the cargo ship. After Cygnus departs
the station in July, it will climb to a higher altitude, around 500 km,
and eject them into space.

Sensor complement: (Imaging Polarimeter)

HARP will be the first US imaging
polarimeter in Space. Polarization measurements are used because the
technique provides new information on aerosol and cloud properties and
their interaction. HARP design is an advance over POLDER’s
(POLarization and Directionality of the Earth's Reflectances) filter
wheel system. The HARP polarimeter will provide full cloudbow
retrievals from a small area (< 4 km x 4 km from space).

Cloud and aerosol processes
influence climate change, which affect our oceans, weather, ecosystems,
and society. The largest impediments to estimating climate change
revolve around a lack of quantitative information about aerosol
forcing, insufficient understanding of aerosol-cloud processes, and
cloud feedbacks in the climate system. The climate community requires
new observations and a better understanding of aerosol and cloud
processes to narrow climate change estimate uncertainties. The aerosol
community requires a multi-wavelength, multi-angle imaging polarimeter
with the wide FOV imaging heritage of the POLDER mission and the high
accuracy promised by the APS (Aerosol Polarimetry Sensor).
Unfortunately, APS was lost when the Glory mission failed to reach
orbit.

An imaging polarimeter with
hyperangular capability can make a strong contribution to
characterizing cloud properties, especially ice clouds. Because of
their sensitivity to thin cirrus clouds, non-polarized multi-angle
measurements can be used to provide climatology. Adding polarization
and increasing the number of observation angles provides a much clearer
picture of cloud droplet distribution, adding size and width
measurements to the currently measured effective radius. The
combination of hyperangular polarized measurements and short-wave
infrared channels (2.1 µm) should also provide enough constraints
to determine important characteristics of cloud ice crystals. In the
coming decades, it will be important to have an imaging polarimeter
with the capability to characterize both aerosols and clouds.
Highly-capable, small, and versatile, HARP is designed to meet the
needs of both the aerosol and cloud communities.

The HARP payload, a
hyperangular imaging polarimeter that can see Earth from multiple
viewing angles, 4 wavelengths, and three polarization angles was
developed and is being built at LACO (Laboratory for Aerosol, Clouds
and Optics) in the Physics Department at UMBC with support from JCET
(Joint Center of Earth Systems and Technology) and NASA/GSFC (Goddard
Space Flight Center). The HARP science algorithms will be developed in
collaboration between UMBC and STC (Science and Technology
Corporation). The main characteristics of the HARP payload are
described in Table 1. 11)12)

• One hyper-angular channel with up to 60 viewing angles per pixel at 670 nm (for cloudbow measurements)

• Three channels with up to 20 viewing angles per pixel at 440, 550, 670 nm

• Goal of one additional channel with up to 20 viewing angles at 870 nm

• 2.5 km nadir resolution (from 650 km orbit)

• 94 degree FOV in cross-track

• 110 degree FOV in along track

Table 1: Polarimeter specifications

HARP is designed to see how aerosols
interact with the water droplets and ice particles that make up clouds.
Aerosols and clouds are deeply connected in Earth's atmosphere –
it's aerosol particles that seed cloud droplets and allow them to grow
into clouds that eventually drop their precipitation (Figure 3). 13)

This interdependence implies that
modifying the amount and type of particles in the atmosphere, via air
pollution, will affect the type, size and lifetime of clouds, as well
as when precipitation begins. These processes will affect Earth's
global water cycle, energy balance and climate.

When sunlight interacts with aerosol
particles or cloud droplets in the atmosphere, it scatters in different
directions depending on the size, shape and composition of what it
encountered. HARP will measure the scattered light that can be seen
from space. We'll be able to make inferences about amounts of aerosols
and sizes of droplets in the atmosphere, and compare clean clouds to
polluted clouds.

In principle, the HARP instrument
would have the ability to collect data daily, covering the whole globe;
despite its mini size it would be gathering huge amounts of data for
Earth observation. This type of capability is unprecedented in a tiny
satellite and points to the future of cheaper, faster-to-deploy
pathfinder precursors to bigger and more complex missions.

HARP is one of several programs
currently underway that harness the advantages of CubeSats for science
data collection. NASA, universities and other institutions are
exploring new earth sciences technology, Earth's radiative cycle,
Earth's microwave emission, ice clouds and many other science and
engineering challenges.

The HARP polarimeter is fully
programmable and will allow for the selection of different spatial
resolutions and combinations of wavelengths and viewing angles
depending on the science interest and total amount of data to downlink.
The different along track viewing angles from HARP will allows the
observations of targets on the ground from different viewing
perspectives. These different viewing observations of the same target
allow for additional information from the target facilitating the
quantitative retrieval of information from the atmosphere and surface
properties such as the aerosol particle amount, the cloud droplet
sizes, and specific characteristics of Earth’s surface.

Figure 6: Photo of the stripe filter unit (image credit: HARP Team)

Figure 7: HARP calibration (image credit: HARP Team)

Figure 8:
A series of pictures of the coast of California taken during the PODEX
(Polarimeter Definition Experiment) campaign by the PACS multi-angle
imaging polarimeter taken from the NASA ER-2 aircraft (image credit:
NASA)

Legend to Figure 8:
The PACS (Passive Aerosol & Clouds Suite) polarimeter serves as an
airborne simulator for the HARP imaging polarimeter. The different
perspectives in the images emphasize the variation of the reflection of
the sun on the ocean surface as a function of the viewing angle. In
some along track viewing angles this reflection disappears while in
other angles this reflections appear very intensively.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).